With the internet being the way it is, I can certainly understand the compulsion to find out where humanity should draw the line, and incest is something that has been strictly prohibited by "the law of nature." I very much mean the law of right and wrong, and I define it so:

This law was originally called the Law of Nature because people thought that every one knew it by nature and did not need to be taught it. They did not mean, of course, that you might not find an odd individual here and there who did not know it, just as you find a few people who are color-blind or have no ear for a tune. But taking the race as a whole, they thought that the human idea of decent behavior was obvious to every one. And I believe they were right.

I know that some people say the idea of a Law of Nature or decent behavior known to all men is unsound, because different civilizations and different ages have had quite different moralities.

But this is not true. There have been differences between their moralities, but these have never amounted to anything like a total difference. If anyone will take the trouble to compare the moral teaching of, say, the ancient Egyptians, Babylonians, Hindus, Chinese, Creeks and Romans, what will really strike him will be how very like they are to each other and to our own. Think what a totally different morality would mean. Think of a country where people were admired for running away in battle, or where a man felt proud of double-crossing all the people who had been kindest to him. You might just as well try to imagine a country where two and two made five. Men have differed as regards what people you ought to be unselfish to--whether it was only your own family, or your fellow countrymen, or everyone. But they have always agreed that you ought not to put Yourself first. selfishness has never been admired. Men have differed as to whether you should have one wife or four. But they have always agreed that you must not simply have any woman you liked.

But the most remarkable thing is this. Whenever you find a man who says he does not believe in a real Right and Wrong, you will find the same man going back on this a moment later. He may break his promise to you, but if you try breaking one to him he will be complaining "It’s not fair" before you can say Jack Robinson.

All of this said, and having established the existence of a law of nature, a brief look at history can prove that incest is immoral, against nature and something to be detested.

Here is how Wikipdia Defines it:
http://en.wikipedia.org/wiki/Incest

Incest is sexual relations between closely related persons, and can be illegal depending on the jurisdiction [type]. The exact definition, including the nature of the relationship between persons, and the types of sexual activity, vary by country, and by even individual states or provinces within a country. These laws can also extend to marriage between said individuals.

When incest involves an adult and a child, it is a form of child sexual abuse and is illegal in every developed country